The present invention relates generally to apparatus utilized in operations in subterranean wells and, in a preferred embodiment thereof, more particularly provides a retainer valve useful in subsea applications.
Retainer valves are well known in the art. They are commonly used in subsea well testing operations above and in close proximity to subsea test trees. A typical retainer valve is operated (selectively opened and closed) by application of fluid pressure to various lines connected to the retainer valve and extending upward to a rig pressure source.
Generally, in a type of retainer valve known as a "normally closed" retainer valve, a compression spring is used to bias a ball valve portion of the retainer valve toward its closed position. Various lines connected to the retainer valve and extending to a rig pressure source are utilized to operate the retainer valve. A control line is utilized to maintain the ball valve portion of the retainer valve in its open position, that is, fluid pressure in the control line biases the ball valve toward its open position against the biasing force exerted by the spring.
Fluid pressure in a balance line is utilized to assist the spring in forcing the ball valve to its closed position. Such assistance is particularly useful when the ball valve is called upon to seal against a large pressure differential from above, which causes a ball of the ball valve to be pressed tightly against a ball seat of the ball valve when the retainer valve is of the type which seals from above.
Another line, known as a latch line, typically extends to the subsea test tree and is used therein to release a latch, thereby enabling a handling string, from which the retainer valve and subsea test tree are suspended, to be disconnected from a valve section of the subsea test tree, so that the handling string may be retrieved in case of an emergency. The valve section of the subsea test tree typically contains one or more normally closed safety valves which are operable by additional lines extending to the rig pressure source.
Thus, in an emergency, when it is desired to retrieve the handling string, fluid pressure may be applied to the latch line, thereby disconnecting the handling string from the subsea test tree, but leaving the closed valve section of the subsea test tree behind. The valve section may later be retrieved by relatching the latch thereto. The latch may also be unlatched by rotation of the handling string, but it is much more desirable to accomplish the unlatching using fluid pressure in the latch line, in part due to the lines externally disposed about the handling string, which may become entangled or cut if the handling string is rotated.
Some retainer valves use the fluid pressure in the latch line to vent pressure trapped between a closed ball valve of the retainer valve and a closed safety valve of the subsea test tree. This enables the latch to be unlatched much easier, since the trapped pressure is not exerting a large axial force on the latch as it is trying to unlatch, in part due to greatly reduced friction. The safety of the unlatching operation is also increased thereby, since a sudden uncontrolled release of the trapped pressure does not occur as the latch is unlatched. Such uncontrolled release of trapped pressure can actually cause the handling string to be propelled violently upward, particularly when a substantial amount of gas is trapped between the ball valve of the retainer valve and the safety valve of the subsea test tree.
Where pressure in the latch line is used to vent the trapped pressure below the ball valve, the latch line is typically connected to a hydraulic-type bleed-off valve. The bleed-off valve opens a flow passage from the interior of the handling string below the ball valve to the exterior of the handling string before the latch of the subsea test tree unlatches. Fluid pressure in the latch line, thus, opens the bleed-off valve and vents the trapped pressure before the retainer valve is disconnected from the subsea test tree. This is another reason why it is generally preferred to unlatch the latch using latch line pressure, rather than by rotating the handling string. Additionally, the use of fluid pressure in the latch line is less time-consuming than rotating the handling string.
Unfortunately, situations occur wherein it is impossible to operate the retainer valve as described above. For example, a boat may strike, or apply a large pulling force, to a portion of the rig connected to the lines, such as the rig pressure source, thereby disconnecting the lines from the pressure source. As another example, a fire or other catastrophe on the rig may temporarily or permanently prevent application of fluid pressure to the lines as desired.
In situations such as these, it may be desired to retrieve the handling string by unlatching the latch on the subsea test tree. Since the retainer valve and subsea test tree both contain normally closed valves which will close when control line pressure is lost, any fluid pressure existing therebetween when the above situations occur will be trapped when the valves close. Additionally, if fluid pressure cannot be applied to the lines, including the latch line, the trapped fluid pressure between the retainer valve and the subsea test tree cannot be vented and the latch cannot be unlatched by applying pressure to the latch line. Furthermore, the ball valve cannot be opened to vent the trapped pressure into the handling string, because fluid pressure cannot be applied to the control line.
From the foregoing, it can be seen that it would be quite desirable to provide a normally closed retainer valve which has the capability of venting fluid pressure trapped below its closed ball valve when the ability to apply pressure to lines connected thereto is lost, which may be opened and pumped through in order to kill the well when the abilit, to apply pressure to lines connected thereto is lost, and which is capable of operating normally when the ability to apply pressure to the lines is regained. It is accordingly an object of the present invention to provide such a retainer valve.